In recent years, it has been considered to form isolation insulators from a coating film such as a polysilazane (PSZ) film. The coating film has an advantage that it can be reliably filled in isolation trenches having narrow widths and high aspect ratios. When the isolation insulators are formed from the coating film, it is necessary to oxidize and cure the coating film by a heat treatment after forming the coating film. However, the stress in the polysilazane film is largely varied due to film shrinkage in the heat treatment, so that such large cracks as to extend to a semiconductor substrate may be generated in the polysilazane film. Accordingly, to prevent the generation of the cracks, a method of conducting the heat treatment of the polysilazane film at a temperature as low as possible to reduce the stress in the polysilazane film has been studied. However, there is a problem that the oxidation of the polysilazane film is insufficient at low temperature, so that only the isolation insulators poor in insulation performance can be obtained.